1. Field of the Invention
The present invention relates to manufacture of multi-layered board materials, and, more particularly, to manufacture of oriented strand board and the like using an improved resin system.
2. Description of the Prior Art
Oriented strand board (xe2x80x9cOSBxe2x80x9d) is commercially available from a number of companies including J. M. Huber Corporation, Georgia-Pacific Corporation, Louisiana-Pacific, as well as other sources. OSB material generally has multiple layers of wood xe2x80x9cflakesxe2x80x9d or xe2x80x9cstrandsxe2x80x9d bonded together by a resin binder. The flakes are made by cutting logs into thin slices with a knife edge parallel to the length of a debarked log. The cut flakes are broken into narrow strands generally having lengths oriented parallel to the wood grain that are larger than the widths. The flakes are typically 0.01 to 0.05 inches thick, although thinner and thicker flakes can be used in some applications, and are typically, less than one inch to several inches long and less than one inch to a few inches wide.
In the fabrication of oriented strand board, the flakes are first dried to remove water, and are then coated in a blender with a thin layer of binder and sizing agent. The coated flakes are then spread on a conveyor belt to provide a first surface ply or layer having flakes oriented generally in line with the conveyor belt, then one or more plies that will form an interior ply or plies of the finished board is (are) deposited on the first ply such that the one or more plies is (are) oriented generally perpendicular to the conveyor belt. Then, another surface ply having flakes oriented generally in line with the conveyor belt is deposited over the intervening one or more plies having flakes oriented generally perpendicular to the conveyor belt. Plies built-up in this manner have flakes oriented generally perpendicular to a neighboring ply insofar as each surface ply and the adjoining interior ply. The layers of oriented xe2x80x9cstrandsxe2x80x9d or xe2x80x9cflakesxe2x80x9d are finally exposed to heat and pressure to bond the strands and binder together. The resulting product is then cut to size and shipped. Typically, the resin and sizing agent comprise less than 10% by weight of the oriented strand board.
Oriented strand board has a wide variety of uses as a construction material such as for flooring, sheathing, concrete forming, and so forth. Conventional oriented strand board has experienced a problem in that xe2x80x9cflake popsxe2x80x9d tend to appear in the surface layer after the board is exposed to humidity, rain or other sources of moisture. For instance, flake pop problems may occur in flooring applications where the OSB is subjected to pooled water for extended periods of time. The flake pops are defects occurring in the exposed surface of an OSB in which one or more individual wood flakes delaminate from the bulk of the surface layer and curl or project outward from the rest of the surface layer. These flake pops can rise up to several inches above the surface layer and often will become separated from the surface layer leaving an indentation and discoloration in their place. As a consequence, the flake pops create an unsightly, bumpy, and nonuniform exposed surface. Flake pops are unacceptable to many customers. An OSB surface having them typically must be restored by sanding procedures.
In the prior art literature, U.S. Pat. No. 2,473,145 to Hesselbart describes preparation of highly water-soluble, fusible thermosetting phenol-formaldehyde resin solid particles capable of serving as an air-drying tack-free adhesive, which can be used for bonding wood sheets or plies into plywood. The Hesselbart patent does not describe use of the phenol-formaldehyde resin in combination with any other resins such as isocyanates, nor applicability of the descriptions to OSB manufacture.
U.S. Pat. No. 4,944,823 to Stofko describes bonding together wood surfaces using a preferably dry binder formulation formed of a mixture of an isocyanate and a sugar or starch, in which the starch or sugar replaces a quantity of the isocyanate which would normally have been used to thereby reduce manufacturing costs. This patent is directed to formaldehyde-free binder formulations for lignocellulosic materials, and it teaches that the isocyanate reacts with the hydroxy groups in the sugar to form urethane groups.
U.S. Pat. No. 5,047,275 to Chiu describes a powdered resin useful in manufacture of composition boards, such as wafer board and OSB, where the spray dried powdered resin is a phenol-aldehyde chelated with an oxo-boron compound.
None of the prior art mentioned above directly addresses the pop flake problem nor proposes a solution to that problem. Therefore, the provision of technique oriented strand board and the like which would reduce and even prevent the pop flake problem would represent a significant advance in the field.
The present invention relates to the manufacture of a multi-layered board having a stable, smooth, and moisture-resistant surface. To accomplish this and other advantages of the invention, an improved binder system is used in the production of the surface layers of a multi-layered board which combines strength, durability, and excellent resin penetration inside the wood constituents of the board""s surface layers.
Among other things, this invention prevents curled flake failures, or so-called xe2x80x9cflake popsxe2x80x9d, from occurring in the wood surfaces of multi-layered board even after a board""s exposure to moisture or other weathering. As a consequence, this invention achieves and then preserves the superior surface quality of the board product during its handling, storage and when put into service.
The improved binder system used in the production of the board surface layers according to this invention involves contacting wood pieces with a binder composition comprised of a curable powdery phenol-formaldehyde resin and a curable isocyanate resin in the substantial absence of free water to provide a composite mixture that can be formed into a surface layer for assembly and lamination together with other wood and resin composite layers. The term xe2x80x9cfreexe2x80x9d, as used herein, means unreacted, and any moisture content contained by the wood pieces themselves is not considered as xe2x80x9cfreexe2x80x9d water for purposes of this disclosure. The curable isocyanate resin is generally used in liquid form, and preferably as a neat liquid. The term xe2x80x9cneatxe2x80x9d, as used herein, means non-diluted by or non-dispersed in water. Consequently, in one aspect, this invention effectively utilizes an overall non-aqueous binder system.
Advantageously, the powdery aldehyde and isocyanate binder system constituents are simultaneously fed into a mechanical blender from separate feed streams for contacting and mixing with wood pieces introduced to the blender. The curable powdery aldehyde resin particles stick to the surfaces of the wood pieces in the surface layer mix, and then penetrate well inside the curled strands during the blending process of the surface layer, where they become fully cured during later hot pressing and consolidation of a stack of board layers comprising the surface layers.
In one advantageous embodiment of the invention, the manufactured multi-layered board is oriented strand board in which the opposite surface layers are prepared with the above-mentioned non-aqueous binder material and having its strands oriented in the machine or cross-machine direction. Preferably, the surface layers are used in combination with one or more core (interior) layers formed of composites of wood pieces and a suitable binder in general having strands oriented in a direction substantially perpendicular to those in the surface layers or in random directions.
Preferably, the surface layer binder material is formulated such that the powdery curable aldehyde resin comprises phenol-formaldehyde and the curable isocyanate resin comprises an aromatic polyiisocyanate. Further, the binder material used in the surface layers encountered in this invention preferably comprises approximately 10 to 90%, and more preferably 20 to 49%, by weight phenol-formaldehyde and approximately 90 to 10%, and more preferably 80 to 51%, by weight aromatic polyiisocyanate.
The present invention is also directed to the board products themselves provided by the manufacturing techniques described herein. This invention is generally applicable to the manufacture of multi-layered board materials in which the constituent layers or plies are composites of small wood pieces, such as wood strands, flakes, chips, wafers, slivers, or particles, or the like, which are bound together with a binder resin. This invention is especially applicable to the manufacture of oriented strand board (OSB), but it is not limited thereto, as multi-layered waferboards, flake boards, particle boards, and the like, are also encompassed by the invention.
The multi-layered boards manufactured by this invention can be used advantageously as a general construction material for exposed or covered flooring, concrete formers, sheathing walls, and roofs, and so forth. Due to the excellent moisture-resistance imparted by the above-mentioned binder system used in the surface layers, the board products of this invention are well-suited for use as exposed flooring in damp environments.
For purposes of this application, xe2x80x9cpowderyxe2x80x9d, xe2x80x9cpowderedxe2x80x9d and other variants thereof used herein to characterize the phenol-formaldehyde resin mean a flowable mass of fine dry solid particles or a particulate. The term xe2x80x9cgranularxe2x80x9d has the same meaning as powdery for purposes of this application.